i hate fuel pressure!

[RedZ85]

I got the Mallory Comp 110 as recomended to replace the not so impressive Holley Blue pump. MY issues with the blue pump were low fuel pressures after 30 min or so of use. THe mallory is much better in that its QUIET! wow! However, my fuel pressure is still a problem. The pump has an internal bypass that regulated the output pressure by just cyciling the fuel once the calibrated output pressure is reached. Here is my current trouble shooting setup. Fuel tank->Filter (NEW)->Comp 110 fuel pump->fuel line to the engine compartment->a pressure gauge and then blocked off. Thats it. I want to see what the fuel pump is outputing and how constant it is over time. Once i turn the pump on, i get around 8 psi and it holds there. If i leave and come back later (10-40 min) its at 2-3 psi...Whats the deal here? I thought there may be a vacuum or soemthing being created in the tank hindering the pumps pumping efficiency, but i can remove the gas cap which should relieve any vacuum and nothing changes! That means its the pump...BUT ITS BRAND NEW! maybe 1.5 gallons through it at most! Whats going on?? ANY IDEAS?

[grumpyvette]

you NEED a fuel pressure regulator with a BYE-PASS CIRCUIT from the regulator at the carbs location back to the tank, dead head regulators will NOT maintain a constant pressure and running a electric pump without useing fuel constantly will USUALLY cause the pump to heat up and LOOSE some efficiency,in fact they comonly cavatate if heated up, trying to pump the same fuel constantly, the flow thru a bye-pass regulator with its seperate fuel return line to the tank allows the fuel to circulate and pull heat from the pump,the better quality, return style regulators are vastly more effective

 

ideally all lines are 3/8" minimum but 1/2" id is far better

the electric fuel pump would ideally be mounted lower than the tank and as far back as practicable INSIDE the frame rails for crash protection, with a hot lead to the electric pump that only supplies electric power ,if you have oil pressure and the ignition keys in the on possition, as electric pumps PUSH fuel,far more efficiently than they PULL fuel

as you don,t want the electric pump running if the engines NOT running.

the fuel filter should be mounted for easy access,as you should change it frequently.

the regulator should be mounted as close to the carb as easy access allows, with the return line back to the tank, ideally larger than the feed line for zero resistance to fuel flow

keep in mind the by-pass style regulators work by allowing all fuel flowing above a certain threshold pressure you set the regulator too,to return to the tank, usually thats set at 4.5-6 psi for carbs[/b] [/i]

keep in mind that the pressure sensor only makes tthe electrical connection once theres 5 psi of oil presure in the blocks oil passages

"Ok G.V., need your input.

I intend to wire up 1 electric pump through an oil pressure switch (GM thought of it first thats how I got the idea). If the car sits dorment for a few weeks and the fuel evaporates in the carb the engine could be hard to start. Granted, you should be able to crank 5 psi long enough to fill the carb and start the engine but this adds wear and tear to the starter. Which of the following solutions would you recommend if any?

(1) Bypass the fuel pump circuit and power it through the start circuit so the pump powers up during engine cranking. This would need a diode so the start circuit is not back fed through the fuel pump circuit after engine start up which complicates the circuit.

(2) Wire in a toggle switch that powers the pump before cranking. Once engine starts, kill the toggle and allow the oil pressure circuit take over. The switch positions would be marked START (hot)and RUN (open). In a (heaven forbid) severe collision, the toggle could be activated (unlikely but possible) and defeat the safety built in to the oil pressure power up circuit.

What do you think?"

 

I would NEVER use a manual toggle switch,on an electric fuel pump they have a tendency to get thrown into the wrong possition at times, causeing all kinds of potential problems ranging from flooded carbs,engine stalling, dead batterys, burned out pumps etc.

 

if you want an over ride switch for the fuel pressure youll want a BUTTON (normally open contacts)that connects the circuit only while the spring loaded button is MANUALLY HELD in the depressed position, similar to the older cars starter circuit buttions, thus the circuit works with or without the button depressed if the engine has oil pressure,and you have the OPTION to pre-prime the carb by MANUALLY depressing and holding the over ride button, momentarily ,sending the electricity to the electric pump to pre -prime the carb. but the fuel,system still works either way

 

 

THE DEAD HEAD STYLE REGULATOR

works with a spring on a valve that allows the valve to open once the DIFFERANCE IN PRESSURE between the sides of the regulator valves fuel lines has changed

think of it as a door that has 7-10psi on the feed side and you want lets assume 5.5 psi at the carb

youll need to understand that the dead head regulator works on the DIFFERANCE in pressure between the feed and use sides so having 10psi on the feed and 7psi on the use side is exactly the same as having 12psi on the feed and 9psi on the use side to that style regulator

as the fuel pump fills the line it eventually (fractions of a second )reaches the point where theres a voluum of fuel past the valve with enought pressure to allow BOTH the SPRING and the fuel pressure past the valve to close the valve untill the fuel is reduced to the point that the SPRING and the remaining fuel pressure/voluum beyond the valve can not hold the valve closed and the valve is force open and held open untill, that diffearance in pressure is restored. now lets launch the car hard, the pump that had maintained 8-10 psi to the regulator, 5.5 psi past the valve and the spring in the regulator is now fighting the fuel in the line feeding the regulators enertia, and the sudden drop in pressure as the throttle drops full open in the carb,what the pump sees is the full 8-10 psi or MORE the regulator sees a sudden drop off to near zero and it opens wide, if the fuel pumps able too it tends to flood the fuel bowl for a second then the valve slams shut, untill the pressure drops off as you hit each gear the cycle repeats, the result is a surge in pressure and a rapid drop off in voluum then a rapid flood of fuel that rapidly cycles as you go down the track

if you had a accurate fuel pressure sensor at the carb youll see a rapidly cycling pressure/flow

if some crud gets stuck in the valve it cant close and your carb FLOODS OUT, because it must fully close every few fractions of a second to work correctly

 

the far superior, bye-pass style regulator functions in a totally differant manor

assuming the same set-up but you replace the regulator with a bye-pass style regulator, the bye-pass regulator works by opening a valve too a much lower pressure path for the fuel to return to the tank,,the open fuel return line. anytime the pressure exceeds the 4.0-5.5 psi,youve set it to, so the fuel line to the carb can only see a max at that 4.0-5.5 psi. now the pumps sitting there potenially supplying at 8-10psi just like before, but it can never exceed 4.0-5.5 psi because the bye pass regulator bleeds of any and all excess flow voluum,the pump supplies. but lets look at your launch, if the pressure drops to 6, or 7 psi nothing changes at the carb, if it increases to 10 or 12 psi, nothing changes at the carb,if it drops to to 4.0-5.5 psi that you set the regulator bye-pass circuit too or less the valve to the bye pass line will close but thats seldom a problem, it the sudden changes in pressure and over pressures that happen when you suddenly change the fuel flow required or the (G)loads on the system that potentially screw things up,the bye-pass regulator style regulator isolates the carb and maintains the desired 4.0-5.5 psi to the carb FAR MORE CONSISTANTLY

now lets assume the spring get weak over time or the adjustment gets set at 4 psi in error, with the bye-pass style youll probably never notice ,if you had a accurate fuel pressure sensor at the carb youll see a rock steady pressure/flow

should some crud get stuck in the valve and it cant close NOT MUCH HAPPENDS, because its normally OPEN not closed

if you check youll see MOST EFI systems are BYE-PASS regulated designs also due to control and relieability issues

 

but on the dead head the cycle just gets about 20% more erratic and more frequent in the cycles, further weakening the spring over time

 

 

 

btw your fuel pump tends to run under less stress and run cooler with a bye-pass style regulator also

[John Scott]

http://centuryperformance.com/fuel.asp

Dead heads require a bigger rated pump than return style, hp for hp. They don't call them dead heads for nothing One source recommends -6 or 3/8 lines up to 375 hp, 550 with -8. Make sure the rest of your system is up to the task.Running a return style regulator is a pain, but worth the effort. Your pump will thank you by only working hard when it needs to, your pressure is stable, fuel cooler, and you can concentrate on the next problem. John

[dr_hunt]

Not to mention that those holley pumps are rated just above worthless junk.

[RedZ85]

I have an oil pressure switch and all that installed. Should i redo all the fuel lines and stick in 3/8 lines with a 3/8" return line?? i think the tube running inside the tank is only 5/16. Would there be any advantage in running a 3/8" line after a 5/16" tank feed? Also, should i go with steel braid fuel hose for everything? As for the regulator, i was looking at this one from summit:

 

http://static.summitracing.com/global/images/prod/large/maa-4309.jpg

 

Its a mallory MAA-4309 3-12 PSI return style regulator. Its got 1 inlet and 3 outlet for a return and dual carb inputs.

[Alf]

i have that exact regulator, worked good. right now im in the process of changing to a mechanical fuel pump so im not using it anymore. its only been on my car for less than a year, ill sell it for $40 if your interested.

[John Scott]

Good regulator. The picture shows it being the one with the reference port. I'm using similar, but 4 port, with -8 in and out then dual -6to the carb. I think mine was rated to 22 psi. 1/1 rise with boost. I just plug the vacuum/boost port. I really don't know what the stock lines are cabable of, so I won't chime in other than make sure your return is of equal or larger than supply. I'm pretty sure NHRA requires solid or braided lines, so its a good move. John

[76brkracer]

check volts after awhile sometimes it will drop.

I use the blue pump but i just make 1/4 mile passes.